In conventional TDM telecommunication systems, designed for the concurrent transmission of a multiplicity of messages over respective channels between two stations or terminals, each channel is provided with an analog/digital converter or coder at its transmission end and a complementary reconverter or decoder at its receiving end, the converter including a generator of a ramp voltage of constant negative slope whose starting amplitude is proportional to the message sample to be transmitted; a counter of clock pulses measures the time required by the ramp voltage to drop to a predetermined reference level whereby the pulse count is proportional to the amplitude of the sample. With a message signal ranging between a lower voltage limit V.sub.min and an upper voltage limit V.sub.max, the range V.sub.max - V.sub.min is divided into 2.sup.n quantum steps so that the digitized sample can be represented by a word of n bits.
If the highest signal frequency has a magnitude of 4KHz, as is usual in a telephone system, a cadence of one sample per half-cycle of this frequency corresponds to a sampling period T.sub.z = 125.mu.s. With an accommodation period T.sub.a = 5.mu.s provided for the storage of each new sample, the available digitization interval T.sub.x = T.sub.z - T.sub.a is 120.mu.s. For n = 12 we have a bit rate of ##EQU1##
A pulse-code-modulation (PCM) system operating at such high speeds requires the use of relatively expensive semiconductors whose energy consumption is high.